The Bright Empires series by Stephan Lawhead 6. The Skin Map 7. The Bone House 8. The Spirit Well
This is books 1-3 of a 5 book series. The last two haven't been published yet. Kit Livingstone one day takes a shortcut through a London street, and meets his great-grandfather, who introduces him to travel through ley lines, which can take a person to any point and time in history, or just around the world (never the future). When Kit tries to show his girlfriend that he's telling the truth, he loses her. What follows is an adventure through various times and places as he looks for both her and the Skin Map.
The books are a bit confusing at times (after all, it is about time travel, which is never linear). Book 1 ended with "How did that person get there?!" It doesn't follow just Kit, but also several other travelers. But for the most part it's an enjoyable read.
"You need me. You wouldn't be much of a hero without a villain. And you do love being the hero, don't you. The cheering children, the swooning women, you love it so much, it's made you my most reliable accomplice." -- Lex Luthor to Superman, Question Authority, Justice League Unlimited
